Solid state light emitting devices based on crystallographically relaxed structures

ABSTRACT

The present invention discloses a method for manufacturing a solid state light emitting device having a plurality of light-sources, the method comprising the steps of: providing a substrate having a growth surface; providing a mask layer on the growth surface, the mask layer having a plurality of openings through which the growth surface is exposed, wherein a largest lateral dimension of each of said openings is less than 0.3 μm and wherein the mask layer may comprise a first mask layer portion and a second mask layer portion, having the same surface area and comprising a plurality of openings wherein the first mask layer portion exhibits a first ratio between an exposed area of the growth surface and an unexposed area of the growth surface, and wherein the second mask layer portion exhibits a second ratio between an exposed area of the growth surface and an unexposed area of said growth surface, the second ratio being different from the first ratio; growing a base structure on the growth surface in each of the openings of the mask layer; and growing at least one light-generating quantum well layer on the surface of each of the base structures.

FIELD OF THE INVENTION

The present invention relates to solid state light emitting devices andto a method for manufacturing such solid state light emitting devices.

BACKGROUND OF THE INVENTION

Solid state light emitting devices such as light emitting diodes (LEDs)and solid state lasers are used in a wide range of applications fromconventional lighting systems to optical communication systems. Inparticular, nitride based LEDs enable the use of LEDs for generallighting purposes. However, emission efficiency for such devices dropsconsiderably for wavelengths above 480 nm. A way to compensate this isto use blue emitting LEDs in such a way that part of the emitted bluelight is converted to green-yellow light by interaction with aphosphorescent material. However, phosphorescent conversion suffers fromseveral conversion loss mechanisms during the transition from a higherenergy radiation to lower energy radiation resulting in limitedconversion efficiency while also the emission peak shape is broadened.Therefore, LEDs emitting directly in the green to red parts of thevisible spectrum would provide considerable benefits such as eliminatedconversion losses and improved color tunability.

For example, gallium nitride (GaN) based light sources can be adapted toshift the emission wavelength towards the red end of the visiblespectrum. WO2008/078297 discloses a method for manufacturing a GaN-basedsemiconductor light emitting device configured to emit multiplewavelengths of light. This is achieved by forming a plurality of postsfrom a mask layer having a plurality of openings, where each postcomprises a light emitting layer disposed between an n-type and a p-typeregion. The emitted wavelength is controlled by the diameter of thepost.

SUMMARY OF THE INVENTION

In view of the aforementioned prior art, it is an object of the presentinvention to provide an improved method for fabrication of solid statelight emitting devices, and in particular an improved method forfabricating solid state light emitting devices enabling fabrication ofdevices emitting wavelengths in the green to red part of the visiblespectrum.

According to a first aspect of the present invention, it is thereforeprovided a method for manufacturing a solid state light emitting devicehaving a plurality of light-sources, the method comprising the steps of:providing a substrate having a growth surface; providing a mask layer onthe growth surface, the mask layer having a plurality of openingsthrough which the growth surface is exposed, wherein a largest lateraldimension of each of said openings is less than 0.3 μm and wherein themask layer comprises a first mask layer portion and a second mask layerportion, having the same surface area and comprising a plurality ofopenings wherein the first mask layer portion exhibits a first ratiobetween an exposed area of the growth surface and an unexposed area ofthe growth surface, and wherein the second mask layer portion exhibits asecond ratio between an exposed area of the growth surface and anunexposed area of said growth surface, the second ratio being differentfrom the first ratio; growing a base structure on the growth surface ineach of the openings of the mask layer; and growing at least onelight-generating quantum well layer on the surface of each of the basestructures.

The term “solid state light emitting devices” should in the presentcontext be understood as semiconductor-based light emitting devices suchas photoluminescent devices, LEDs, laser diodes or vertical cavitysurface emitting lasers (VCSELs). A light source should in the presentcontext be understood as each individual structure which emits lighteither in an electrically driven device through direct emission or in apassive device through a photoluminescent reaction followingphotoexcitation.

The light-generating quantum well (QW) layer is a thin layer of amaterial having a lower energy band gap than the surrounding materials,thereby forming a potential well. Light is generated when chargecarriers recombine over the bandgap and the size of the bandgapdetermines the wavelength of the emitted light. Charge carriers can beprovided either through electrical injection in an electrically drivendevice or through photoexcitation in a passive device. By growing thelight-generating quantum well layer on an at least partially relaxedbase structure it is possible to achieve material compositions, andthereby band gap energies, in the quantum well layer not possible toachieve on a non-relaxed surface. Thus, providing at least partiallyrelaxed base structures promotes growth of a quantum well layer withdesirable light-emitting properties not achievable on a non-relaxedsurface. In particular, it is possible to grow quantum wells havingemission peaks in the red part of the visible spectrum. It is equallypossible to grow multiple quantum wells stacked on the base structure inorder to manufacture light emitting devices such as LEDs, laser diodesand VCSELs. Furthermore, limiting the size of the openings to achievethe growth of relaxed structures also enables the use of non-latticematched substrates which would otherwise give rise to stress relatedproblems commonly occurring when growing larger structures or continuousfilms.

The present invention is based on the realization that the properties oflight-sources based on crystallographically relaxed structures,epitaxially grown on a growth surface in openings of a certain size, canbe controlled by controlling the relative size and separation distanceof the openings, and in particular by controlling the ratio between theexposed growth surface area and the mask layer area.

As base structures which are at least partially relaxed allow subsequentgrowth of a light-generating quantum well layer with propertiesdifferent from what is possible to grow on strained material, it isdesirable to achieve such crystallographically relaxed base structures.Provided that a largest lateral dimension of each of the openings isless than 0.3 μm, local surface relaxation will reduce or eliminate thestress that otherwise would result from a lattice mismatch between thelattice constant of the growth surface and lattice constant of the basestructure. The largest lateral dimension of f.i. a polygon is thelargest diagonal, i.e. the largest line segment connecting two differentnon-sequential corners of the polygon. The largest allowable size ofeach of the openings to achieve relaxed base structures is for aselected material combination determined by material parameters such asYoung's modulus and lattice constant.

The wavelength of the light emitted from a quantum well is related tothe quantum well thickness which in turn is a result of the growthconditions used when growing the quantum well layer. As a precursor isprovided, commonly in gas or vapor form, the precursor can be assumed touniformly reach the entire surface of the wafer in a common rotatingwafer configuration during deposition. Since growth only takes place onthe base structures and not on the mask layer surface, precursormaterial deposited on the mask layer surface migrates towards theopenings containing the base structures, there contributing to quantumwell growth. Thus, the ratio between exposed and unexposed growthsurface area determines the amount of precursor material available togrow the quantum well layer. As a result, a portion of the wafer where alarger fraction of the growth surface is exposed results in thinnerquantum wells than on a portion of the wafer where a smaller fraction ofthe growth surface is exposed.

Hence, the size of the opening in combination with the separation willthen determine the relative growth rate and thereby the thickness of QWsgrown on a particular base structure. The combination of opening sizeand separation distance is thus a strong means to tune QW thickness andthereby emission color from a specific light source.

A substrate acting as a carrier may be provided. The substrate mayadvantageously be a wafer in a conducting material enabling contactingof the backside of the substrate. More specifically, the substrate maycomprise a material selected from the group consisting of GaN, sapphire,silicon, SiC, ZnO, ScN, TiN, HfN, AlN, ZrB₂, HfB₂, NbB₂, BP, GaAs, GaP,LiGaO₂, NdGaO₃, LiAlO₂, ScMgAlO₄, garnet and spinel.

A growth surface may be provided on an upper surface of the substratewhere the growth surface promotes growth of the desired relaxed basestructure. In the present context a growth surface may advantageously bea surface suitable for epitaxial growth of a group III-Vsemiconductor-based material, more specifically the growth surface maybe suitable for epitaxial growth of GaN based materials. Forelectrically driven devices, a GaN based n-doped growth layer may beused.

A mask layer may be arranged on top of the growth surface and openingsmay be created in the mask layer exposing the growth surface. The masklayer may advantageously be insulating as is the case for a SiO₂ basedmaterial. The mask layer may also be selected from a wide range ofinsulating materials such as SiN_(x), TiO₂, ZrO₂ or similar oxides,nitrides or carbides.

Epitaxial growth of the base structures starts at the exposed growthsurface, meaning that no growth takes place on the surface of the masklayer. Consequently the openings in the mask layer define where basestructures are grown. The main purpose of the base structure is tofunction as a base for subsequent growth of a light-generating quantumwell layer which may be grown as a continuous film on the surface of thebase structure. For electrically driven solid state lighting devices,the base-structure is preferably n-doped.

It should also be noted that for a photoluminescent device, quantumwells are not strictly required. In a photoluminescent device, the basestructures may be the light emitting structures, although also in suchdevices quantum wells may be used for additional control of the lightemitting properties.

According to one embodiment of the present invention, the plurality ofopenings are of substantially the same size and the distance betweenadjacent openings is greater in the second mask layer portion than inthe first mask layer portion. Hereby, it is possible on a single waferto tailor the quantum well properties to be different in differentportions of the device while using substantially the same opening sizein the first and second portions. Needless to say, there may be morethan two mask layer portions having mutually different distances betweenopenings.

This provides for a highly flexible way of simultaneously producing onthe same surface a combination of light sources emitting differentwavelengths, while being able to use an optimal opening size so thatrelaxed base structures can be grown in all openings if desired.

Put in slightly different words, the density of openings ofsubstantially the same size may be different in the first and secondmask layer portions. For example, the openings may be arrangedsubstantially regularly, and the pitch may vary between the twoportions.

The distance between openings may, for example, be at least 10% greaterin the second mask layer portion than in the first mask layer portion,whereby a substantial color difference can be achieved while stillgrowing the light-generating quantum well layers on relaxed basestructures. By locally varying separation distance of the openingsand/or opening size, the emitted wavelength can be tuned locally. Bydoing this over relatively large areas of a device (large enough to becontacted separately) the device becomes segmented and thereby colortunable. On the other hand, by randomly or quasi-randomly varying eitheror both of opening size and separation, thus varying QW thickness, ahighly uniform mixture of light-sources with different wavelengths maybe obtained, which can be an advantage in applications that are highlydemanding from a uniformity point of view. Furthermore, the possibilityto tailor emission in combination with the possibility to emitwavelengths covering the visible spectrum, as a result of using relaxedbase structures, makes it possible to produce LED devices emitting whitelight.

According to another embodiment of the present invention, each openingin the plurality of openings has a polygon shape, wherein at least oneside of the opening is aligned substantially parallel to acrystallographic orientation of the growth surface;

The shape of the opening will have an effect on the lattice structure ofthe crystal planes that constrain the polyhedron grown from the polygonshaped opening. Different crystal planes in the base structure maypossess different growth properties, thereby resulting in a differentmaterial composition or layer thickness of the grown light-generatingquantum well layer. A different material composition or thickness of thequantum well may lead to a shift in emission wavelength. The differencesin quantum well properties corresponding to different crystal planes ofthe base structure may be relatively small and will in that case mainlylead to an apparent broadening of the total emission peak. A broaderemission peak results in a more continuous emission spectrum, thusleading to a better color perception of the emitted light. The effect ofdifferent crystal planes with different properties will as an example beevident for polyhedrons shaped as truncated pyramids, where a quantumwell grown on the top surface is likely to exhibit not only a differentmaterial composition, but also a notably different growth rate comparedto a quantum well grown on the side walls of the pyramid. Additionally,for pyramids, the total growth area of the structure (initially theopening in the mask) grows once the pyramid shape starts forming andmore so when the pyramid is overgrowing the mask, implying that therelative growth rate perpendicular to the surface starts decreasing.This can be exploited as this effect will occur earlier and to a largerextent with small sized holes at a relatively small separation distance.

A different growth rate of the QWs at different planes is one way inwhich the color of emitted light can be tuned more strongly as this willresult in QWs of different thickness. The thickness of a quantum welldirectly determines the extent of quantum confinement, which in turn (incombination with material composition and strain induced by latticemismatch) determines the wavelength of the emitted light.

Furthermore, the alignment of the openings may advantageously beselected so that all sides of the base structures are equivalent withrespect to the growth surface which results in a higher degree ofuniformity over the device area.

Additionally, a controlled shape and alignment of the opening mayadvantageously lead to a base structure with fewer crystallographicdefects, thereby reducing the risk of non-radiative recombinationcenters which reduce the efficiency of the device.

In the case of strong misalignment between the sides of the openings andthe underlying crystallographic structure, the grown structure willpartly align to various crystallographic directions. For example, amisaligned quadratic opening may result in a grown structure which isfacetted, meaning that a polyhedron with 6 or 8 sides may be grown.However, some misalignment may be allowed as the grown structuresinherently align to the preferred growth directions.

In one embodiment of the present invention, the shape of the opening mayadvantageously be hexagonal. By aligning the sides of the grown basestructures to a desired underlying crystallographic orientation, makingall sides of the structure equivalent, it is possible to achieve ahigher uniformity and thereby a better defined emission wavelength,which may be favorable when monochromatic emission is desired. The shapeof the opening may equally well be triangular, rectangular or any otherpolygon depending on the crystallographic structure of the growthsurface.

According to one embodiment of the present invention, the method forfabricating a solid state light emitting device may further comprise thestep of providing a first contacting structure on the light-generatingquantum well layer of each of the base structures and a secondcontacting structure electrically contacting the base structure.

According to one embodiment of the present invention the firstcontacting structure may advantageously comprise a charge carrierconfinement layer arranged on the surface of the light-generatingquantum well layer followed by a conducting layer on the surface of theconfinement layer. The charge carrier confinement layer is part of ahetero structure forming the quantum well, where the function of theconfinement layer is to define one of the boundaries of the quantumwell, providing an energy barrier between the quantum well and theneighboring material. The opposite quantum well boundary is formed bythe base structure. As an example, the charge carrier confinement layermay be an electron-blocking layer and the conducting layer may comprisea p-doped hole conduction layer with contacts arranged on the conductinglayer. The device can be contacted as planar grown LEDs, i.e. byapplying proper contacting layers to both the n-doped growth layer andthe p-doped top layer.

The contacts may be formed on opposite sides of the device or they mayboth be on the same side of the device. When both contacts are arrangedon the same side, the device may be formed either with transparentcontacts and mounted such that light is extracted on the same side asthe contacts. Alternatively, the contacts may be reflective and mountedas a flip chip in which case light is extracted from the opposite sideof where the contacts are arranged.

In one embodiment of the present invention, the step of providing themask layer may comprise the steps of depositing a mask layer material onthe growth surface and selectively removing mask layer materialaccording to a predefined pattern to form the aforementioned openings.

According to one embodiment of the present invention, patterning of themask layer may advantageously be done by nano-imprinting. By using apatterning method such as surface conformal nano-imprinting, patterningcan be done on wafer scale in a single process step. Additionally,problems with wafer bending associated with other lithography methodscan be reduced or even avoided. A pattern may advantageously beimprinted in deformable silica provided in the form of sol-gel derivedSiO₂, there forming a plurality of depressions corresponding to apredefined imprinting template. After nano-imprinting, a thin residuallayer of silica may remain at the bottom of the depressions. Theresidual layer may preferably be removed by selectively etching the masklayer material with respect to the underlying growth layer where theremoval, for example, can be done using reactive ion etching (RIE).Other patterning methods are also available such as stepper lithography,e-beam lithography and holographic interference lithography. Suitablemask removal methods would be used for the respective lithographymethods.

According to a second aspect of the present invention, it is provided asolid state light emitting device comprising: a substrate having agrowth surface; a mask layer on the growth surface, the mask layerhaving a plurality of openings, wherein a largest lateral dimension ofeach of said openings is less than 0.3 μm and wherein the mask layercomprises a first mask layer portion and a second mask layer portion,having the same surface area and comprising a plurality of openingswherein the first mask layer portion exhibits a first ratio between theopening area and the mask layer area, and wherein the second mask layerportion exhibits a second ratio between the opening area and the masklayer area, the second ratio being different from the first ratio; an atleast partially crystallographically relaxed base structure grown on thegrowth surface in each of the openings of the mask layer; and alight-generating quantum well layer grown on the surface of each of thebase structures.

Effects and features of this second aspect of the present invention arelargely analogous to those described above in connection with the firstembodiment. However, some additional features will be discussed.

According to an embodiment of the solid state light emitting deviceaccording to the present invention, the base structures mayadvantageously protrude above the mask layer. The grown base structuresare not necessarily limited by the thickness of the mask layer, on thecontrary, they may instead protrude above and extend beyond the openingsin the mask layer. For continued growth after the structures haveprotruded above the mask layer the structures may also extend in alateral direction. Allowing extended growth provides another possibilityto tune the size and geometry of the base structures and quantum wellsin addition to the geometrical disposition of the openings discussed inrelation to the first aspect of the present invention.

According to one embodiment of the present invention, the growth surfacemay advantageously comprise a GaN or InGaN growth layer arranged on thecarrier substrate. From the growth layer a GaN or InGaN base structuremay be grown followed by an InGaN quantum well layer. It would also bepossible to use other material combinations, preferably from the III-Vgroup of semiconductor-based materials, and more preferably from thesubset of nitride based III-N materials.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail with reference to the appended drawings showing exemplaryembodiments of the invention, wherein:

FIG. 1A is a flow-chart schematically illustrating a portion of anexemplary manufacturing method according to an exemplary embodiment ofthe present invention;

FIG. 1B is a flow-chart schematically illustrating a portion of anexemplary manufacturing method according to an exemplary embodiment ofthe present invention;

FIGS. 2 a-h schematically illustrate the steps of the method illustratedby the flow-chart in FIG. 1;

FIGS. 3 a-c schematically illustrate an intermediate step in thefabrication method for making a solid state light emitting deviceaccording to an exemplary embodiment of the present invention; and

FIGS. 4 a-b schematically illustrates alternative light source patternsfor solid state light emitting device according to various embodimentsof the present invention.

DETAILED DESCRIPTION

In the following detailed description, various embodiments of the methodfor manufacturing solid state light emitting devices according to thepresent invention are mainly discussed with reference to a method basedon growth of relaxed semiconductor structures as a base for lightemitting diodes.

It should be noted that this by no means limits the scope of the presentinvention which is equally applicable to passive devices emitting lightthrough photoluminescent emission. Manufacturing methods usingvariations of the processing steps described below are also possible. Asan example, other mask patterning methods such as photolithography ore-beam lithography may be used. Furthermore, the method is equallyapplicable for other material combinations, primarily comprisingmaterials from the III-V group of semiconductor-based material. Also,growth methods may be selected from a range of methods enablingepitaxial growth, for instance, MBE may be used instead of MOVPE.

An exemplary method according to an embodiment of the present inventionwill now be described with reference to the flow-chart shown in FIG. 1Aand FIG. 1B together with FIGS. 2 a-2 h schematically illustrating thedevice in different stages of the manufacturing process.

In a first step 101, a substrate 201 having a growth surface 204 isprovided as shown in FIG. 2 a. A suitable substrate can be a sapphire ora doped silicon carbide (SiC) wafer. As an example, a GaN layer 203 isdeposited on the surface of the sapphire wafer 202 to form a growthsurface 204 as shown in FIG. 2 a. Growth surfaces consisting of a socalled buffer layer of relaxed GaN obtained by hetero-epitaxial growthof GaN on commonly used substrates are well-suited. At least the top ofthis GaN-layer is n-doped for fabricating an electroluminescent device.

In the next step 102, a mask layer 205 is provided as illustrated inFIG. 2 b. A deformable precursor layer to the mask layer is deposited onthe GaN growth surface. The deformable precursor is in this embodimentsol-gel derived SiO₂.

In the following step 103, the mask layer 205 is patterned. Patterningis done by imprinting the precursor layer over the entire area of thewafer in a single step using surface conformal nano imprinting (SCIL),resulting in depressions in the form of squares in the silica layer asshown in FIG. 2 c. The largest lateral dimension of each of thedepressions is less than 0.3 μm. As a result of imprinting, there may bea thin residual layer of silica at the bottom of the depressions. Theremaining silica at the bottom of the depressions can be removed throughreactive ion etching, forming openings 206 exposing the GaN growthsurface 204. The etching of the silica is preferably selective withrespective to the GaN growth surface 204.

In the subsequent step 104 of growing the base structures 207, as isschematically illustrated in FIG. 2 d, InGaN base structures 207 aregrown on the exposed GaN growth surface 204 in each of the openings 206of the mask layer 205. InGaN growth may, for example, be done in ametalorganic vapor phase epitaxy (MOVPE) reactor and the In content isdetermined by the trimethylindium (TMI) to triethylgallium (TEG)precursor ratio. A variation in In content may also be achieved byadapting the temperature during growth. The epitaxially grown InGaN basestructures 207 only nucleate in the openings 206 at the GaN surface 204and no growth takes place on the surface of the silica mask layer. Dueto the limited dimensions of the openings 206, the resulting InGaN basestructures 207 have a relaxed crystal structure. The shape of the basestructures 207, when grown in the manner as described in the presentembodiment using square openings, is a square pyramid. Depending ongrowth time the top of the pyramid might be flat. The shape of the topis not critical to the function of the light emitting device and shapessuch as conventional pyramids are equally possible. The base structures207 may or may not protrude above the mask layer 205 depending on themask layer thickness and growth time of the base structures 207. In thepresent embodiment, base structure growth is stopped before coalescence,but it would in principle be equally possible to fabricate lightemitting devices from coalesced structures, provided that the basestructures remain crystallographically relaxed.

In the next step 105, illustrated in FIG. 2 e, a thin light-generatingquantum well layer 208 is grown on the surface of the base structures207. The epitaxially grown quantum well layer 208 only grows on the basestructure 207 and no growth takes place on the surface of the mask layer205. In this embodiment, the quantum well layer 208 comprises InGaNhaving a higher In content than the base structure 207. The relaxedcrystal structure of the base structure 207 leads to the incorporationof a higher In content in the quantum well layer 208 compared to what isincorporated on non-relaxed base structures under the same conditions.By increasing the In content, the band gap is reduced sufficiently so asto allow the emission of green or red light. In another embodiment, thebase structures 207 can be made of GaN with an InGaN quantum well forfabrication of light sources emitting blue light.

The next step 106, illustrated in FIG. 2 f, is the deposition of acharge carrier confinement layer 209. In the present embodiment, thecharge carrier confinement layer 209 comprises an aluminum galliumnitride (AlGaN) electron blocking layer.

In the next step 107 illustrated in FIG. 2 g a conducting layer 210 isdeposited. The deposition of a conducting layer 210 comprises depositionof a p-doped InGaN layer and followed by a p⁺⁺-doped InGaN layer.

The final step 108 in the method according to an embodiment of thepresent invention is illustrated in FIG. 2 h where a contact layer 211is deposited in the form of an electrically conductive contact materialsuch as ITO (Indium Tin Oxide) or Pd/Au. Conventional processing cansubsequently be used to define the p-contacts and n-contacts. Contactsto the n-doped side of the device may be achieved either through holesetched in the top-layers or via substrate removal and contacts at then-doped side. The derived solid state lighting device itself may becontinuous or segmented to enable differentiated contacting toindividual (base) structures over the device area.

FIG. 3 a illustrates device having a pattern 301 where the size andseparation distance of the openings 206 in the mask layer varies overthe surface of the device. By varying the size and separation distanceof the openings 206 as outlined it is possible to tailor the resultinglight sources to emit different wavelengths on the same wafer withoutadding any process steps. In general, having a smaller opening to maskratio, as illustrated in the top part 302 of FIG. 3 a, result in athicker quantum well layer which in turn gives emission at a higherwavelength compared to an area with a larger opening to mask ratio 303.As an example, by combining multiple wavelengths it is possible tofabricate devices emitting white light. As another example, bysegmenting and separately contacting areas emitting differentwavelengths, a color tunable device can be made.

FIG. 3 b illustrates the openings 206 in the mask layer 205 exposing thegrowth surface 204 prior to growth of base structures.

FIG. 3 c schematically illustrates a device having openings of the samesize but where the separation distance of the openings is different indifferent regions of the device. In particular, in a first region (302)the distance between the openings are larger than in a second region(303) which leads to a different density of openings.

FIG. 4 a schematically illustrates a possible shape of the openings inthe mask layer where a hexagonal shape 402 enables alignment alongspecific crystal directions in the underlying growth surface. As anexample, two schematic crystal directions of the growth surface, c₁ andc₂, are shown. It is also shown that a hexagonal opening may be alignedso that it has sides (404, 405) substantially parallel to the crystaldirections. By aligning the sides of the grown base structures to adesired underlying crystallographic orientation, making all sides of thestructure equivalent, it is possible to achieve a higher uniformity andthereby more controlled emission properties. An even higher degree ofuniformity can be obtained when the hexagonal openings 402 are packed ina hexagonal pattern 403 as illustrated in FIG. 4 b, as then also thedistances between adjacent hexagons can be chosen constant.

As an example, aligning a hexagonal configuration of openings along thelow-index orientation on a GaN surface implies that all opening wallswill be either along the (10-10) or (11-20) direction.

As another example, triangular openings oriented along the majorlow-index crystallographic axis on the C-plane of GaN will lead to welldefined crystallographic growth of equivalent crystal planes.

As yet another example, when growing structures on the M-plane, A-planeor R-plane of sapphire, all exhibiting a rectangular symmetry, at leasttwo different sets of planes are initially grown and the ratio betweenthe planes can be tuned by selecting the size shape and orientation ofthe opening.

The person skilled in the art realizes that the present invention by nomeans is limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within thescope of the appended claims. For example, the method according to thepresent invention may also be used to fabricate vertical cavity emittinglasers (VCSELs) and laser diodes (LDs). Additionally, LEDs having beenfabricated according to the method described above but with differentmaterial compositions are equally possible. Also, by omitting the n-typeand p-type doping as well as the contacting layers, the resultingstructure is well suited to act as a photoluminescent emitter with mostof the properties with respect to wavelength tuning and selection asdescribed above. Such a material could be directly illuminated by asuitable UV or blue emitting light source or through a waveguidestructure. Furthermore, the basic principle of the present invention mayalso be applied to the fabrication of group III-V basedsemiconductor-based devices in general by providing a surface for growthhaving the desired properties related to lattice parameters and strainlevels.

The invention claimed is:
 1. A method for manufacturing a solid statelight emitting device having a plurality of light sources, the methodcomprising the steps of: providing a substrate having a growth surface;providing a mask layer on said growth surface, said mask layer having aplurality of openings through which said growth surface is exposed,wherein a largest lateral dimension of each of said openings is lessthan 0.3 μm and wherein said mask layer comprises a first mask layerportion and a second mask layer portion, each comprising a plurality ofopenings exposing said growth surface, wherein said first mask layerportion exhibits a first ratio between an exposed area of said growthsurface and an unexposed area of said growth surface, and wherein saidsecond mask layer portion exhibits a second ratio between an exposedarea of said growth surface and an unexposed area of said growthsurface, said second ratio being different from said first ratio;growing a base structure on said growth surface in each of said openingsof said mask layer; and growing at least one light-generating quantumwell layer on the surface of each of said base structures.
 2. The methodaccording to claim 1, wherein said plurality of openings are ofsubstantially the same size and wherein the distance between adjacentopenings is greater in said second mask layer portion than in said firstmask layer portion.
 3. The method according to claim 1, wherein eachopening in the plurality of openings has a polygon shape, wherein atleast one side of said opening is aligned substantially in parallel to acrystallographic orientation of said growth surface.
 4. The methodaccording to claim 1, wherein said opening is hexagonal.
 5. The methodaccording to claim 1, further comprising the step of providing a firstcontacting structure on the light-generating quantum well layer of eachof said base structures and a second contacting structure electricallycontacting the base structure.
 6. The method according to claim 5,wherein the step of providing a first contacting structure comprises thesteps of: growing a charge carrier confinement layer on the surface ofsaid light-generating quantum well layer; providing a conducting layeron the surface of said charge carrier confinement layer; and providingcontacts to the conducting layer.
 7. The method according to claim 1,wherein said step of providing a mask layer comprises the steps of:depositing a mask layer material on said growth surface; and selectivelyremoving mask layer material to form said openings.
 8. The methodaccording to claim 7, wherein said step of selectively removingcomprises the steps of: patterning said mask layer by producing aplurality of depressions in said mask layer through nano-imprinting; andremoving said mask layer material at the bottom of said depressions byselectively etching said mask layer material with respect to said growthsurface material to form said openings exposing said growth surface. 9.A solid state light emitting device comprising: a substrate having agrowth surface; a mask layer on said growth surface, said mask layerhaving a plurality of openings, wherein a largest lateral dimension ofeach of said openings is less than 0.3 μm and wherein said mask layercomprises a first mask layer portion and a second mask layer portion,each comprising a plurality of openings, wherein said first mask layerportion exhibits a first ratio between the opening area and the masklayer area, and wherein said second mask layer portion exhibits a secondratio between the opening area and the mask area, said second ratiobeing different from said first ratio; an at least partiallycrystallographically relaxed base structure grown on said growth surfacein each of said openings of said mask layer; and a light-generatingquantum well layer grown on the surface of each of said base structures.10. A solid state light emitting device according to claim 9, whereinsaid plurality of openings are of substantially the same size andwherein the distance between adjacent openings is greater in said secondmask layer portion than in said first mask layer portion.
 11. A solidstate light emitting device according to claim 9, wherein each of theplurality base structures has a polygon shape, where at least one sideof said polygon is aligned substantially in parallel to acrystallographic orientation of said growth surface.
 12. A solid statelight emitting device according to claim 9, further comprising a firstcontacting structure arranged on the light-generating quantum well layerof each of said base structures and a second contacting structureelectrically contacting the base structure.
 13. The solid state lightemitting device according to claim 9, wherein each of said basestructures protrudes above said mask layer.
 14. The solid state lightemitting device according to claim 9, wherein at least one of saidgrowth surface and base structures comprises GaN or InGaN.
 15. The solidstate light emitting device according to claim 9, wherein said quantumwell layer comprises InGaN.